Ion mobility spectrometers (IMS) can identify material from a sample of interest by ionizing the material and measuring the time it takes the resulting ions to travel a known distance under a known electric field. Each ion's time of flight can be measured by a detector, and the time of flight is associated with the ion's mobility through a gas. An ion's mobility relates to its mass and collision cross section. Therefore, by measuring the time of flight of an ion in the detector it is possible to infer an identity for the ion. These times of flight may be displayed graphically or numerically as a plasmagram.
Different materials give rise to differently charged ions. Ion mobility spectrometry can be used for screening for traces of contraband such as explosives and narcotics. Narcotics may generally be detectable in an IMS operating in a positive mode, whereas narcotics may be detectable in negative mode. Some chemical weapons agents may be detected in positive mode, and others in negative modes. Some IMS devices therefore include both positive and negative mode cells.
Where space and electrical power resources are not limited the provision of both positive and negative mode IMS cells is unproblematic. In hand-held devices there are constraints on both size and weight of the device. The problem of dielectric breakdown of materials used in IMS power supply units also imposes a voltage related size constraint on IMS apparatus.